1. Field of the Invention
This invention relates to sensorless rotor position monitoring in reluctance machines, particularly in switched reluctance machines.
2. Description of Related Art
The control and operation of switched reluctance machines generally are described in the paper “The Characteristics, Design and Applications of Switched Reluctance Motors and Drives” by J. M. Stephenson and R. J. Blake, which is incorporated herein by reference, delivered at the PCIM'93 Conference and Exhibition held in Nürnberg, Germany, 21–24 Jun. 1993. In that paper the “chopping” and “single-pulse” modes of energization of switched reluctance machines are described for operation of the machine at low and high speeds, respectively.
A typical prior art drive is shown schematically in FIG. 1. This includes a DC power supply 11 that can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply 11 is switched across phase windings 16 of the motor 12, connected to a load 19, by a power converter 13 under the control of the electronic control unit 14. One of the many known converter topologies is shown in FIG. 2, in which the phase winding 16 of the machine is connected in series with two switching devices 21 and 22 across the busbars 26 and 27. Busbars 26 and 27 are collectively described as the “DC link” of the converter. Energy recovery diodes 23 and 24 are connected to the winding to allow the winding current to flow back to the DC link when the switches 21 and 22 are opened. A capacitor 25, known as the “DC link capacitor”, is connected across the DC link to source or sink any alternating component of the DC link current (i.e. the so-called “ripple current”), which cannot be drawn from or returned to the supply. In practical terms, the capacitor 25 may comprise several capacitors connected in series and/or parallel and, where parallel connection is used, some of the elements may be distributed throughout the converter. A resistor 28 is connected in series with the lower switch 22 to provide a current feedback signal. A multiphase system typically uses several “phase legs” of FIG. 2 connected in parallel to energize the phases of the electrical machine.
The performance of a switched reluctance machine depends, in part, on the accurate timing of phase energization with respect to rotor position. Detection of rotor position is conventionally achieved by using a transducer 15, shown schematically in FIG. 1, such as a rotating toothed disk mounted on the machine rotor, which co-operates with an optical, magnetic or other sensor mounted on the stator. A signal, e.g. a pulse train, indicative of rotor position relative to the stator is generated by the sensor and supplied to control circuitry, allowing accurate phase energization. This system is simple and works well in many applications. However, the rotor position transducer increases the overall cost of assembly, adds extra electrical connections to the machine and is, therefore, a potential source of unreliability.
Various methods for dispensing with the rotor position transducer have been proposed. Several of these are reviewed in “Sensorless Methods for Determining the Rotor Position of Switched Reluctance Motors” by W F Ray and I H Al-Bahadly, published in the Proceedings of The European Power Electronics Conference, Brighton, UK, 13–16 Sep. 1993, Vol. 6, pp 7–13, incorporated herein by reference.
Many of these methods proposed for rotor position estimation use the measurement of phase flux-linkage (i.e. the integral of applied voltage with respect to time) and current in one or more phases. Position is calculated using knowledge of the variation in inductance of the machine as a function of angle and current. This characteristic can be stored as a flux-linkage/angle/current table and is depicted graphically in FIG. 3. The storage of this data is a disadvantage as it involves the use of a large memory array and/or additional system overheads for interpolation of data between stored points.
Some methods make use of this data at low speeds where “chopping” current control is the dominant control strategy for varying the developed torque. Chopping control is illustrated graphically in FIG. 4(a) in which the current and inductance waveforms are shown over a phase inductance period. (Note that the variation of inductance is depicted in idealized form.) These methods usually employ diagnostic pulses in non-torque-productive phases. A method suited to low-speed operation is that proposed by N. M. Mvungi and J. M. Stephenson in “Accurate Sensorless Rotor Position Detection in an S R Motor”, published in Proceedings of the European Power Electronics Conference, Firenze, Italy, 1991, Vol. 1, pp 390–393, incorporated herein by reference.
Other methods operate in the “single-pulse” mode of energization at higher speeds. This mode is illustrated in FIG. 4(b) in which the current and inductance waveforms are shown over a phase inductance period. These methods monitor the operating voltages and currents of an active phase without interfering with normal operation. A typical higher speed method is described in International Patent Application WO 91/02401, incorporated herein by reference.
Having to store a two-dimensional array of machine data in order to operate without a position sensor is an obvious disadvantage. Alternative methods have been proposed, which avoid the need for the majority of angularly referenced information and instead store data at one angle only. One such method is described in European Patent Application EP-A-0573198 (Ray), incorporated herein by reference. This method aims to sense the phase flux-linkage and current at a predefined angle by adjusting the diagnostic point in accordance with the calculated deviation away from the desired point. Flux-linkage is estimated by integrating (with respect to time) the measurement of the voltage applied to the phase. Two one-dimensional tables are stored in the preferred embodiment, one of flux-linkage versus current at a referenced rotor angle and another of the differential of flux-linkage with respect to rotor angle versus current. By monitoring phase voltage and current, the deviation away from a predicted reference angle can be assessed, with the aid of the look-up tables, and system operation can be adjusted accordingly. This method has been shown to be reliable, provided that the flux-linkage can be determined with sufficient accuracy whenever required by the position-detecting algorithm. To avoid the flux-linkage integrator drifting (due to unwanted noise in the system and imperfections in the integrator) it is set to zero at the end of each conduction cycle, when the current has fallen to zero and the phase winding is no longer linking any flux. This method is a “predictor/corrector” method, in that it initially predicts when the rotor will be at a reference position, measures parameters of the machine when it believes the reference position has been reached, and uses the results of these measurements to detect error in the prediction and hence take corrective action by adopting a new prediction for the next reference position.
A special mode of operation of switched reluctance machines is the continuous current mode, as disclosed in U.S. Pat. No. 5,469,039 (Ray), which is incorporated herein by reference. In this mode, the winding is re-connected to the supply before the flux, and hence the current, have returned to zero at the end of the energy return period. The phase windings therefore operate with current continuously flowing through them and are always linked by flux. This is an important mode for systems which have to produce high levels of overload output at some points of their operating cycle. Although the efficiency of the drive falls in this mode, it allows specifications to be achieved which would otherwise require a larger machine. However, in this mode there is no opportunity in the phase cycle to reset the integrators at some known point of zero flux and current, since such a point does not exist. It is therefore considered impossible to apply the method disclosed in EP-A-0573198.
Attempts to find a solution to this problem have included schemes which allow the drive to operate in continuous current mode except when the control system judges it essential to re-estimate the position, at which time the continuous current mode is exited, the position estimated, and the drive put back into continuous current. Specifically, this can be done by running the machine in a mode which is predominantly continuous current but drops back into discontinuous current at predetermined intervals to allow positional information to be gained. The technique depends on the speed being virtually constant, which may be approximately true at higher speeds (at which continuous current is usually employed). Nevertheless, a loss of torque is associated with dropping out of continuous current. An alternative method is to operate each phase in continuous current for a given number of cycles, say 10, and then excite the phase for a shorter time on the next cycle such that the current will definitely fall to zero, allowing the integrator to be reset and an accurate estimate of flux-linkage to be made. By interleaving this “short” cycle with the other phases operating in continuous current, the deleterious effect of the loss of torque is mitigated. However, none of these methods is satisfactory, since the loss of torque can render the machine performance unstable and several cycles are required before stability is reached again because the current must be built up over a period in the continuous current mode.